1. Field of the Invention
The present invention relates to a voltage detection circuit for a plurality of cells, such as lithium ion cells or nickel cadmium cells, connected in series to each other in a battery, and to a method of detecting a cell voltage.
2. Description of Related Art
In some cases, so-called cells such as lithium ion cells or nickel cadmium cells are connected in series to each other for use as a battery pack or a battery to supply an electric power to an electronic equipment such as handy-type video tape recorder, electronic camera, portable computer or the like.
A cell is a single unit of a battery. FIG. 1 is a circuit diagram of a conventional cell voltage detection circuit. FIG. 2 shows an example in which a plurality of such cells (1 (1a to 1d) is connected in series to each other and each of them is connected to a cell voltage detector 9. Of these cells, a one indicated with the reference 1a has a highest potential, while a one indicated with the reference 1d has a lowest potential.
FIG. 2 is a circuit diagram of the cell voltage detector 9 included in the cell voltage detection circuit in FIG. 1. The cell voltage detector 9 comprises an operational amplifier 8 having a noninverting terminal 8a and an inverting terminal 8b. For example, the cell 1a having the highest potential has a positive electrode thereof connected to the noninverting terminal 8a of the operational amplifier 8 of the cell voltage detector 9 via a sense line 2 and resistor R1, and a negative electrode thereof connected to the inverting terminal 8b of the operational amplifier 8 via a sense line 3 and resistor R2. Similarly, each of the other cells has positive and negative electrodes thereof connected to an operational amplifier 8 via a sense line and resistor. Thus, the operational amplifier 8 will detect a voltage from the corresponding cell 1 and provide it as its voltage output 11 individually.
As shown in FIG. 2, each voltage detector 9 in FIG. 1 as provided therein a plurality of resistors R1 to R4 for the operational amplifier 8.
The detecting accuracy of the cell voltage detector 9 having the above-mentioned configuration greatly depends upon the accuracy of each of the resistors R1 to R4 and that of the detected voltage output 11. Therefore, a resistor of which the resistance is set with a high accuracy should be used as the resistors R1 to R4.
Also, the detecting accuracy of te voltage detector 9 becomes lower proportionally to a higher common mode voltage. This will be explained with reference to FIG. 3.
The output Vo of the operational amplifier is simply expressed (offset error of the operational amplifier is neglected): EQU Vp=Rb(V1+V2)/(Ra+Rb)
where Vp:
Ra, Rb: Resistance of resistors R1 and R2 PA1 V1: Voltage of cell having highest voltage PA1 V2: Total voltage of other cells PA1 a first input selecting means connected to positive and negative electrodes of each cell to select either the positive or negative electrode of a selected cell and acquire a voltage of the cell; PA1 a second input selecting means connected to the positive and negative electrodes of each cell to select either the positive or negative electrode of the selected cell and acquire a voltage of the cell; PA1 a voltage detecting means for acquiring a detected output voltage from output voltages from the first and second input selecting means; and PA1 a processing means for converting the detected cell output voltage from the voltage detecting means from analog to digital for calculation to thereby determine the voltage of the individual cell. PA1 a voltage detecting means for acquiring a detected output voltage from output voltages from the first and second input selecting means; and a processing means for converting the detected cell output voltage from the voltage detecting means from analog to digital for calculation to thereby provide the voltage of the individual cell, PA1 the method comprising, according to the present invention, the steps of: PA1 selecting an input to the first input selecting means and an input to the second input selecting means to supply from the voltage detecting means a positive detected output voltage to the analog/digital converter of the processing means and calculate an amplification factor of the voltage detecting means based on a known calibrating reference voltage connected to either the first or second input selecting means; PA1 supplying a common mode voltage from a same electrode of a cell to the voltage detecting means via the first and second input selecting means to acquire a first positive detected output voltage indicative of an error due to the common mode voltage supplied to the voltage detecting means; PA1 supplying to a noninverting input terminal of the voltage detecting means a voltage at the positive cell electrode selected by the first input selecting means and to an inverting input terminal of the voltage detecting means a negative cell voltage selected by the second input selecting means, to acquire a second positive detected output voltage; and PA1 calculating as follows by the processing means to determine a cell voltage: EQU Voltage E=(second detected output voltage-first detected output voltage)/voltage amplification factor
Therefore, EQU Vo=Vp-Rd(V2-Vp)/Rc
where Rc., Rd: Resistance of resistors R3 and R4 EQU Assume that Ra=Rc and Rb=Rd (1)
Then, EQU Vo=Rd.multidot.V1/Rc (2)
When V1=OV, the output Vo of the operational amplifier should be O. Actually, however, V1 will not be OV because of the error due to the common mode voltage at the operational amplifier.1
The expression (1) is not practically possible. When the expression (1) is impossible, the output Vo of the operational amplifier has an error proportional to a voltage V2.2
These errors 1 and 2 cause to low the detecting accuracy of the cell voltage detection circuit.
Further, since in the conventional cell voltage detection circuit shown in FIG. 2 needs one cell voltage detector 9 for each cell, it is much complicated.